Grippers are mechanical devices characterized by one or more jaws that are reciprocally rotated or translated so that the working end of each jaw is moved together or apart by a motive device such as an electric motor, electric solenoid, pneumatic piston, or other fluid powered actuator. In many cases, the jaws of the gripper are responsible for transferring the force of the gripper to a workpiece such that the workpiece may then be moved, and/or rotated from one machine work station to another. The loading of the workpiece can fail in one of two ways. The first failure mode is characterized in that no workpiece is clamped between the jaws of the gripper. The second failure mode is characterized in that more than one workpiece is clamped between the jaws of the gripper. Each condition is undesirable because it often causes damage to the machine or causes machine down time.
It is known to use electronic sensors or switches to produce an output to signal whether loading failure has occurred or not. Current methods sense the relationship between a sensing object connected to the linear driving member, such as a piston rod, and the body of the gripper to determine one of the two failure conditions. When the workpiece is thin, the differential between the sensing object position when the gripper is clamped on one piece and its position on two pieces is small relative to the sensing object length in the direction of travel. In this case two sensors or switches are needed. A first sensor or switch is needed to sense whether there is at least one workpiece present, and a second sensor or switch is needed to sense that there are at least two workpieces present. The use of two sensors or switches in this way disadvantageously increases the cost, size, and weight of the gripper.
What is needed in the art is a way to reliably use a single sensor or switch to distinguish between the condition of having a gripper properly clamped on a single workpiece, and the condition of having a gripper improperly clamped on no workpiece or on two or more workpieces.